On the reorientation transition of ultra-thin Ni/Cu(001) films

The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within a Heisenberg spin model. Using a modified mean field formulation it is possible to calculate properties of magnetic thin films with non-integer thicknesses. This is especially important for the reorientation transition in Ni/Cu(001), as there the magnetic properties are a sensitive function of the film thickness. Detailed phase diagrams in the thickness-temperature plane are calculated using experimental parameters and are compared with experimental measurements by Baberschke and Farle (J. Appl. Phys. 81, 5038 (1997)).Comment: 7 pages(LaTeX2e) with one figure(eps), accepted for publication in JMMM. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm

Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition

The influence of uniaxial anisotropy and the dipole interaction on the direction of the magnetization of ultra-thin ferromagnetic films in the ground-state is studied. The ground-state energy can be expressed in terms of anisotropy constants which are calculated in detail as function of the system parameters and the film thickness. In particular non-collinear spin arrangements are taken into account. Conditions for the appearance of a spin reorientation transition are given and analytic results for the width of the canted phase and its shift in applied magnetic fields associated with this transition are derived.Comment: 6 pages, RevTeX

Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic Films

The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within Heisenberg spin models. Analytic expressions for the temperature dependent anisotropy are derived from which it is seen that the reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition. Detailed phase diagrams in the temperature-thickness plane are calculated.Comment: 6 pages(LaTeX2e), one figure(eps), accepted for publication in JMM

Influence of magnetic fields on the spin reorientation transition in ultra-thin films

The dependence of the spin reorientation transition in ultra-thin ferromagnetic films on external magnetic fields is studied. For different orientations of the applied field with respect to the film, phase diagrams are calculated within a mean field theory for the classical Heisenberg model. In particular we find that the spin reorientation transition present in this model is not suppressed completely by an applied field, as the magnetization component perpendicular to the field may show spontaneous order in a certain temperature interval.Comment: 11 pages(LaTeX2e), 6 figures(eps), submitted to Phil. Mag. B. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm

Reorientation transition of ultrathin ferromagnetic films

We demonstrate that the reorientation transition from out-of-plane to in-plane magnetization with decreasing temperature as observed experimentally in Ni-films on Cu(001) can be explained on a microscopic basis. Using a combination of mean field theory and perturbation theory, we derive an analytic expression for the temperature dependent anisotropy. The reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition as with increasing temperature the influence of the uniaxial anisotropies is reduced at the surface and is enhanced inside the film.Comment: 4 pages(RevTeX), 3 figures (EPS

Are WC9 Wolf-Rayet stars in colliding-wind binaries?

We present results from a spectroscopic search for massive companions to dust-making Galactic WC9 stars as a step to testing the paradigm that dust formation in these systems requires colliding winds to produce over densities. We find evidence for OB companions to the WC9 stars WR 59 and WR 65, but not WR 121 or WR 117. We identify lines of N III-V and possibly N II in the spectrum of WR 88, one of the few Galactic WC9 stars which do not make circumstellar dust, and suggest that WR 88 is a transitional WN-WC9 object and less evolved than the other WC9 stars. On the other hand, the possible identification of a strong emission line at 4176A in the spectrum of WR 117 with Ne I suggests that this star is more evolved than other WC9 stars studied.Comment: 4 pages, 3 figures, contribution to "Massive Stars and High-Energy Emission in OB Associations"; JENAM 2005, held in Liege (Belgium

Sheared Ising models in three dimensions

The nonequilibrium phase transition in sheared three-dimensional Ising models is investigated using Monte Carlo simulations in two different geometries corresponding to different shear normals. We demonstrate that in the high shear limit both systems undergo a strongly anisotropic phase transition at exactly known critical temperatures T_c which depend on the direction of the shear normal. Using dimensional analysis, we determine the anisotropy exponent theta=2 as well as the correlation length exponents nu_parallel=1 and nu_perp=1/2. These results are verified by simulations, though considerable corrections to scaling are found. The correlation functions perpendicular to the shear direction can be calculated exactly and show Ornstein-Zernike behavior.Comment: 6 pages, 3 figure

Monte Carlo Simulation of Ising Models with Dipole Interaction

Recently, a new memory effect was found in the metamagnetic domain structure of the diluted Ising antiferromagnet $Fe_x Mg_{1-x} Cl_2$ by domain imaging with Faraday contrast. Essential for this effect is the dipole interaction. We simulate the low temperature behavior of diluted Ising-antiferromagnets by a Monte Carlo simulation considering long range interaction. The metamagnetic domain structure occurring due to the dipole interaction is investigated by graphical representation. In the model considered the antiferromagnetic state is stable for an external magnetic field smaller than a lower boundary $B_{c1}$ while for fields larger than an upper boundary $B_{c2}$ the system is in the saturated paramagnetic phase, where the spins are ferromagnetically polarized. For magnetic fields in between these two boundaries a mixed phase occurs consisting of ferromagnetic domains in an antiferromagnetic background. The position of these ferromagnetic domains is stored in the system: after a cycle in which the field is first removed and afterwards applied again the domains reappear at their original positions. The reason for this effect can be found in the frozen antiferromagnetic domain state which occurs after removing the field.Comment: Latex, 10 pages; 3 postsript-figures, compressed tar-file, uuencoded, report 10109

Critical behavior of the Random-Field Ising model at and beyond the Upper Critical Dimension

The disorder-driven phase transition of the RFIM is observed using exact ground-state computer simulations for hyper cubic lattices in d=5,6,7 dimensions. Finite-size scaling analyses are used to calculate the critical point and the critical exponents of the specific heat, magnetization, susceptibility and of the correlation length. For dimensions d=6,7 which are larger or equal to the assumed upper critical dimension, d_u=6, mean-field behaviour is found, i.e. alpha=0, beta=1/2, gamma=1, nu=1/2. For the analysis of the numerical data, it appears to be necessary to include recently proposed corrections to scaling at and beyond the upper critical dimension.Comment: 8 pages and 13 figures; A consise summary of this work can be found in the papercore database at http://www.papercore.org/Ahrens201